Probe heads of the switching type rely on a bearing or seat which, with high precision, fixes the null position of a deflectably movable part that carries the probe element of the probe head. The deflectable part of the probe head is pressed or pulled by a pre-loading device such as a spring; the spring provides defined force for seating in this bearing, which is generally an isostatic three-point bearing. Probe heads of this type are illustratively described in U.S. Pat. No. 4,177,568.
Probe heads of the switching type are used predominantly in an installed vertical orientation. In this case, the force of the spring which loads the seat engagement acts in the direction of gravity and acts uniformly on the three bearing points.
It is also already known to mount switching probe heads on a so-called turn-swing device, providing selective orientation of a probe via orthogonally related axes of rotation; and then to position the probe-pin in different orientations which correspond to workpiece bores to be measured. However, this is only possible if the pre-loading force to seat in the null position is much greater than the weight of the deflectable probe-pin. Otherwise, during a swinging motion of the probe head, the probe-pin may leave the null position determined by the bearing; for example, in a horizontal orientation, forces act transversely to the bearing and can unseat the probe-pin and its carrier, out from the bearing.
On the other hand, if greater pre-loading force is provided to assure null-position seating when the probe-pin is horizontally oriented, there will necessarily be greater work-contacting forces, as well as an increase in bearing friction. Increased friction significantly impairs the precision with which the probe-pin returns into its null position after performing a workpiece-contacting procedure. Furthermore, high workpiece-contacting forces are generally undesired for many measurement tasks.
Although this problem can still be compensated by increasing the pre-loading force within certain limits, for a probe head of relatively small diameter and mounting a probe-pin of only slight mass, such compensation is no longer possible for switching heads of larger construction.